In recent years, research on spatial multiplexing technology using characteristic properties of propagation space has been pursued to investigate increases in transmission capacity. Among the spatial multiplexing technologies, a technology in which a different signal is transmitted from each of a plurality of transmitting antennas and signals are received by a plurality of receiving antennas to separate a received signal is called a multi-input multi-output (MIMO) technology. The MIMO technology is a system that transmits and receives different signals by multiplexing them in space depending on transfer functions on a plurality of propagation paths and different signals are transmitted separately by different antennas, but if there is a strong antenna correlation in which the transfer functions are similar among the plurality of propagation paths, signals cannot be separated, leading to degradation of characteristics.
Thus, for example, according to a technology described in Japanese Patent Application Laid-Open No. 2003-204317, conditions of the propagation paths are acquired and depending on the acquired conditions, different signals are transmitted, different frequencies are used, or the same signals are transmitted from a plurality of antennas.
FIG. 14 is a diagram showing an outline configuration of a conventional radio transmission device. The radio transmission device transmits radio signals, for example, by the OFDM method. Since the OFDM method can reduce an influence of multi-path delay spread in high-speed digital signal transmission by using multiple carriers and inserting guard intervals, the method has attracted attention as a promising next-generation mobile broadband radio access system. Here, an OFDM signal is obtained by multiplexing signals of a plurality of orthogonal sub-carriers and will be described below by taking a case of two antennas as an example.
As shown in FIG. 14, a radio transmission device 100 has a system 1 transmitting a transmission signal A and a system 2 transmitting a transmission signal B. The system 1 includes an encoding part 101, a sub-carrier modulation part 102, an inverse fast Fourier transform (IFFT) part 103, a slot assembly part 104, a frequency conversion part 105, and an antenna 106. The system 2 includes an encoding part 111, a sub-carrier modulation part 112, an inverse fast Fourier transform (IFFT) part 113, a slot assembly part 114, a frequency conversion part 115, and an antenna 116. Further, the transmitter 100 includes a carrier frequency control part 121, a transmission signal switching part 122, and an overall control part 123.
Next, adaptive control for the conventional radio transmission device having such a configuration will be described. Here, a case in which four channels (frequency bands) from channel 1 (CH1) to channel 4 (CH4) can be used as available frequencies is taken as an example for the description below. In FIGS. 15 to 17, an antenna #1 is the antenna 106 of the system 1 and an antenna #2 is the antenna 116 of the system 2.
The radio transmission device 100 can take three radio communication modes. The first one is spatial multiplexing, that is, as shown in FIG. 15, different information (transmission signal A≠transmission signal B) is transmitted from the two antennas 106 and 116 using the same frequency. The second one is frequency multiplexing, that is, as shown in FIG. 16, different information (transmission signal A≠transmission signal B) is transmitted from the two antennas 106 and 116 using different frequencies. The third one is spatial diversity, that is, as shown in FIG. 17, the same information (transmission signal A=transmission signal B) is transmitted from the two antennas 106 and 116 using the same frequency.
More specifically, if, for example, an error detection result is good, that is, propagation path conditions are good, spatial multiplexing is performed by transmitting different information (transmission signal A≠transmission signal B) from the two antennas 106 and 116 using the same frequency, as shown in FIG. 15. In the example shown in FIG. 15, the same idle channel (CH3) is used for multiplexing different transmission signals A and B to transmit from the antenna 106 of the system 1 and the antenna 116 of the system 2 respectively while avoiding frequencies (channels) of CH1, CH2, and CH4 where interference waves exist, that is, that are already allocated to other users. At this point, a reception operation using the frequency (the frequency of CH 3 in the example of FIG. 15) used by the transmitter 100 is performed on a receiver side.
If, for example, the error detection result is not good, that is, propagation path conditions are not good, frequency multiplexing is performed by transmitting different information (transmission signal A≠transmission signal B) from the two antennas 106 and 116 using different frequencies, as shown in FIG. 16. In the example shown in FIG. 16, out of two channels CH2 and CH3 that are idle, one channel (CH2) is used for transmission of the transmission signal A from the antenna 106 of the system 1 and the other channel (CH3), which is different from that of the system 1, is used for transmission of the transmission signal B, which is different from that of the system 1, from the antenna 116 of the system 2 while avoiding frequencies (channels) of CH1 and CH4 where interference waves exist, that is, that are already allocated to other users. At this point, a reception operation using the frequencies (the frequency of CH2 for the system 1 and that of CH3 for the system 2 in the example of FIG. 16) of the systems used by the transmitter 100 is performed on the receiver side.
If, for example, the error detection result is extremely bad, that is, propagation path conditions are so bad that different information cannot be transmitted from a plurality of antennas, spatial diversity is performed by selectively transmitting the same information (transmission signal A=transmission signal B) from the two antennas 106 and 116 using the same frequency, as shown in FIG. 17. In the example shown in FIG. 17, the same idle channel (CH3) is used to perform spatial diversity transmission of the same transmission signal (transmission signal A=transmission signal B) from the antenna 106 of the system 1 and the antenna 116 of the system 2 while avoiding frequencies (channels) of CH1, CH2, and CH4 where interference waves exist, that is, that are already allocated to other users. At this point, a reception operation using the frequency (the frequency of CH3 in the example of FIG. 17) used by the transmitter 100 is performed on the receiver side.
Data transmitted from the transmitter described above is arranged, for example, as shown in FIG. 18 or 19. Here, the case of 30 sub-carriers is shown and, if the same signal is transmitted from the antenna 1 and the antenna 2, as shown in FIG. 18, the data is allocated to data sub-carriers in ascending order of frequency respectively. If, on the other hand, different data is transmitted from each antenna in spatial multiplexing, as shown in FIG. 19, the first to the 30th data are allocated to the antenna 1 and the 31st to the 60th data are allocated to the antenna 2. Patent Document 1: Japanese Patent Application Laid-Open No. 2003-204317